feat(windows): per-app screen-audio sharing (include/exclude)

Adds "only selected apps" and "all apps except selected" screen-audio
modes to the Windows client, alongside the existing entire-desktop path.

Per-app capture uses WASAPI process loopback (AUDCLNT_ACTIVATIONTYPE_
PROCESS_LOOPBACK) via ProcessLoopbackCapture, mixed by ProcessAudioMixer
and fed to the core through vc_stream_feed_pcm (external_feed=1 so the
core skips its own loopback device).

Init must pass AUDCLNT_STREAMFLAGS_LOOPBACK | EVENTCALLBACK |
AUTOCONVERTPCM; the LOOPBACK flag is what makes the virtual endpoint
deliver rendered audio (without it every buffer is flagged SILENT) and
AUTOCONVERTPCM resamples the app's native format to 48k s16.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.Interop/Structs.cs

@@ -46,6 +46,9 @@ internal struct VcStreamDescNative
     public IntPtr DeviceId; // unused by vc_stream_start today — device selection is a
                              // separate vc_set_input_device call; always IntPtr.Zero here.
     public IntPtr Label;
+    // Mirrors vc_stream_desc::external_feed. When 1, the core skips its own WASAPI loopback
+    // and the caller feeds PCM via StreamFeedPcm (per-app capture path on Windows).
+    public int ExternalFeed;
 }
 
 [StructLayout(LayoutKind.Sequential)]

clients/windows/VoiceCat.Interop/VoiceCatClient.cs

@@ -186,6 +186,26 @@ public sealed class VoiceCatClient : IDisposable
         }
     }
 
+    /// <summary>
+    /// Like <see cref="StartStream"/> but sets <c>external_feed = 1</c> so the core skips its
+    /// own WASAPI loopback. The caller is responsible for feeding PCM via
+    /// <see cref="StreamFeedPcm"/>. Used by the Windows per-app capture path.
+    /// </summary>
+    public (VcResult Result, uint StreamId) StartStreamExternalFeed(VcStreamKind kind, string label)
+    {
+        nint labelPtr = Marshal.StringToCoTaskMemUTF8(label);
+        try
+        {
+            var desc = new VcStreamDescNative { Kind = kind, DeviceId = 0, Label = labelPtr, ExternalFeed = 1 };
+            var r = NativeMethods.vc_stream_start(_handle.DangerousGetHandle(), in desc, out uint streamId);
+            return (r, streamId);
+        }
+        finally
+        {
+            Marshal.FreeCoTaskMem(labelPtr);
+        }
+    }
+
     public VcResult StopStream(uint streamId) =>
         NativeMethods.vc_stream_stop(_handle.DangerousGetHandle(), streamId);